1. Field of the Invention
The present invention relates to an electric power apparatus, namely a variable inductor of the type comprising a magnetic core having three limbs, primary or input winding means supplied with alternating current, and a direct current control circuit.
2. Description of the prior art:
Conventionally, the primary winding means of such a variable inductor comprise at least one winding supplied with an alternating current which induces an alternating magnetic flux of a same density within two of the three limbs of the magnetic core. On the other hand, the control circuit is supplied with a direct current which induces a direct current magnetic flux of a same density within these two limbs. The alternating and direct current fluxes assist in one of the two limbs while they oppose in the other, and vice versa depending on the positive or negative value of the alternating current. The function of the direct current magnetic flux induced in each of the two limbs is to saturate more or less deeply the magnetic core for thereby determining the permeability of the latter to the alternating flux and thus the impedance of the primary winding means. This impedance may therefore be varied by modifying the amplitude of the direct current of the control circuit so as to modify the density of the direct current magnetic flux induced in the two limbs. A plurality of systems have been proposed to adjust the amplitude of this direct current whereby a desired operating characteristic of the variable inductor is obtained, some of these systems rectifying the alternating current of the primary winding means for supplying the control circuit with this rectified current.
These known variable inductors have the drawback that their operating characteristic is very sensitive to any variation in the intrinsic properties of the material constituting the magnetic core and in the construction of this core, to heating or to the slightest displacement in the magnetic core, and also to the effect related to the frequency. Moreover, such inductors of the prior art do not allow to obtain an operating characteristic which would provide an optimum range of variation of the alternating current in the primary winding means and therefore of the reactive power of the variable inductor in response to a slight variation of the voltage between the terminals of these primary winding means, and that at a given voltage level. Such an operating characteristic would be very useful for an application of the variable inductor for example to the regulation of alternating voltage.